Method and apparatus for optical temperature measurement

Abstract

A temperature probe and a method for using said probe for temperature measurements based on changes in light absorption by the probe. The probe comprises a first and a second optical fiber that carry light to and from the probe, and a temperature sensor material, the absorbance of which changes with temperature, through which the light is directed. Light is directed through the first optical fiber, passes through the temperature sensor material, and is transmitted by a second optical fiber from the material to a detector. Temperature-dependent and temperature-independent factors are derived from measurements of the transmitted light intensity. For each sensor material, the temperature T is a function of the ratio, R, of these factors. The temperature function f(R) is found by applying standard data analysis techniques to plots of T versus R at a series of known temperatures. For a sensor having a known temperature function f(R) and known characteristic and temperature-dependent factors, the temperature can be computed from a measurement of R. Suitable sensor materials include neodymium-doped boresilicate glass, accurate to ±0.5° C. over an operating temperature range of about -196° C. to 400° C.; and a mixture of D 2 O and H 2 O, accurate to ±0.1° C. over an operating range of about 5° C. to 90° C.

What is claimed is: 
     
       1. A method for measuring temperature, said method for use with a source of light, a detector, and a temperature sensor having at least one isobestic wavelength, said sensor absorbing light in a temperature-dependent manner within a characteristic wavelength range, said method comprising the steps of: transmitting light from said light source through said temperature sensor to said detector;   measuring the intensity I b  of light transmitted through said sensor at a first wavelength, said first wavelength being outside said characteristic wavelength range;   measuring the intensity I I  of light transmitted through said sensor at said isobestic wavelength;   measuring the intensity I A  of light transmitted through said sensor at an analytic wavelength, said analytic wavelength being within said characteristic wavelength range;   computing the quantities A I  =-ln (I I  /I b ) and A A  =-ln (I A  /I b );   computing the ratio R AI  =A A  /A I  ; and   computing said temperature, whereby said temperature may be determined from a function of said ratio, said function formed by (a) while maintaining said sensor at a known temperature, measuring said intensity I b , measuring said intensity I I , and measuring said intensity I A ,   (b) computing said quantities A I  and A A  from said measurement at said known temperature.   (c) computing said ratio R AI ,   (d) repeating steps (a) through (c) with said sensor maintained at different known temperatures to produce a plurality of temperature measurements and a plurality of ratio measurements, and   (e) obtaining said function by correlating said pluralities of temperature and ratio measurements.     
     
     
       2. The method as recited in claim 1, further comprising the step of passing said transmitted light through a filter, said filter selectively transmitting light at said first wavelength, said isobestic wavelength, and said analytic wavelength. 
     
     
       3. The method as recited in claim 1, further comprising the initial step of passing light from said source through a filter, said filter selectively transmitting light at said first wavelength, said isobestic wavelength, and said analytic wavelength. 
     
     
       4. The method as recited in claim 1, wherein said temperature is approximately related to said ratio R AI  by the equation T=AR AI  +BR AI   2  +C, wherein the quantities A, B and C are constants for said sensor, and wherein said function-obtaining step further comprises determining said quantities A, B and C from said pluralities of temperature and ratio measurements. 
     
     
       5. A temperature probe for use with a source of light and a detector, said probe comprising: a temperature sensor having a substantially optically-transparent housing, and   a mixture of D 2  O and H 2  O contained within said housing,     said sensor transmitting at least a portion of the light incident thereon in relation to the temperature of said sensor, said transmitted light having a characteristic factor and a temperature-dependent factor, the ratio of said temperature-dependent factor to said characteristic factor being a known function of temperature;   first means for transmitting light from said source to said temperature sensor; and   second means for transmitting light from said temperature sensor to said detector.   
     
     
       6. The temperature probe as recited in claim 5 wherein said first light-transmitting means further comprises a first optical fiber, and said second light-transmitting means further comprises a second optical fiber, wherein said temperature sensor has a substantially flat surface and opposing curved surface, and wherein said probe further comprises a reflector positioned adjacent to said curved surface so that light entering said sensor from said first optical fiber is reflected by said reflector to said second optical fiber.   
     
     
       7. The temperature probe as recited in claim 5, wherein said mixture has at least one isobestic wavelength, said mixture absorbing light in a temperature-dependent manner within a characteristic wavelength range, further comprising a filter, said filter selectively transmitting light at said isobestic wavelength, a base wavelength, and an analytic wavelength, said base wavelength being outside said characteristic range and said analytic wavelength being within said characteristic range. 
     
     
       8. The temperature probe as recited in claim 5 wherein said first light-transmitting means further comprises an optical fiber, said fiber having a first end in optical communication with said source and a second end in optical communication with said temperature sensor, said fiber bent near said second end at an angle of approximately 180°. 
     
     
       9. The temperature probe as recited in claim 5, wherein said second light-transmitting means further comprises an optical fiber, said fiber having a first end in optical communication with said temperature sensor and a second end in optical communication with said detector, said fiber bent near said first end at an angle of approximately 180°. 
     
     
       10. The temperature probe as recited in claim 5, wherein said mixture contains approximately 90 wt. % D 2  O and approximately 10 wt. % H 2  O. 
     
     
       11. A method for measuring temperature in a region, said method comprising the steps of: transmitting light through a temperature sensor located in said region, said sensor absorbing a portion of said light in relation to the temperature of said sensor;   measuring the intensity of said light transmitted through said temperature sensor, said transmitted light having a characteristic factor and a temperature-dependent factor, the ratio of said temperature-dependent factor to said characteristic factor being a known function of temperature, said function found by (a) measuring the intensity of light transmitted through said sensor at a known temperature,   (b) computing said characteristic factor and said temperature-dependent factor from said measurement,   (c) computing the ratio of said temperature-dependent factor to said characteristic factor,   (d) repeating steps (a) through (c) with said sensor maintained at different known temperatures to produce a plurality of temperature measurements and a plurality of ratio measurements, and   (e) obtaining said function by correlating said pluralities of temperature and ratio measurements;     computing said temperature-dependent factor;   computing said characteristic factor;   computing said ratio of said temperature-dependent factor to said characteristic factor; and   computing said temperature using said ratio and said function.   
     
     
       12. The method as recited in claim 11, wherein said sensor absorbs light in a temperature-dependent manner within a characteristic wavelength range and absorbs light in a temperature-independent manner at an isobestic wavelength, wherein said measuring step further comprises: measuring the intensity I b  of light transmitted through said sensor at a first wavelength, said first wavelength being outside said characteristic wavelength range;   measuring the intensity I I  of light transmitted through said sensor at said isobestic wavelength;   measuring the intensity I A  of light transmitted through said sensor at an analytic wavelength, said analytic wavelength being within said characteristic wavelength range; and   wherein said ratio-determining step further comprises computing the ratio R AI  =A A  /A I , where A A  =-ln (I A  /I b ) and A I  =-ln (I I  /I b ).   
     
     
       13. The method as recited in claim 12, wherein said temperature is approximately related to said ratio R AI  by the equation T=AR AI  +BR AI   2  +C, wherein the quantities A, B and C are constants for said sensor, and wherein said function-obtaining step further comprises determining said quantities A, B and C from said pluralities of temperature and ratio measurements. 
     
     
       14. The method as recited in claim 12 wherein said temperature is approximately related to said ratio R AI  by the equation T=AR AI  +BR AI   2  +C, wherein the quantities A, B and C are constants for said sensor, and wherein said function-obtaining step further comprises determining said quantities A, B and C from said pluralities of temperature and ratio measurements; and wherein said temperature-computing step farther comprises computing said temperature from said equation.   
     
     
       15. The method as recited in claim 11, wherein said measuring step further comprises measuring the absorbance spectrum of said sensor, and wherein said ratio-computing step further comprises: computing the first derivative S' of said absorbance spectrum;   separating said first derivative into a characteristic absorbance C 1  and a temperature-dependent absorbance C 2  ;   computing the quantities E 1  =S'·C 1  and E 2  =S'·C 2  ; and   computing the ratio R 21  =E 2  /E 1 .   
     
     
       16. The method as recited in claim 14, wherein said temperature is approximately related to said ratio R 21  by the equation T=AR 21  +BR 21   2  +C, wherein the quantities A, B and C are constants for said sensor, and wherein said function-obtaining step further comprises determining said quantities A, B and C from said pluralities of temperature and ratio measurements. 
     
     
       17. The method as recited in claim 14, wherein said temperature is approximately related to said ratio R 21  by the equation T=AR 21  +BR 21   2  +C, wherein the quantities A, B and C are constants for said sensor, and wherein said function-obtaining step further comprises determining said quantities A, B and C from said pluralities of temperature and ratio measurements; and wherein said temperature-computing step further comprises computing said temperature from said equation.   
     
     
       18. The method as recited in claim 11, wherein said transmitting step further comprises transmitting said light through a sensor made of rare earth-doped glass. 
     
     
       19. The method as recited in claim 11, wherein said transmitting step further comprises transmitting said light through a sensor made of neodymium-doped glass. 
     
     
       20. The method as recited in claim 11, wherein said transmitting step further comprises transmitting said light through a fluid sensor, said sensor including a mixture of D 2  O and H 2  O.